Conductive inks, particularly carbon-based inks, have been widely used in the manufacture of conducting elements in printed circuits and sensor electrodes. Other major markets for conductive inks include emerging applications, such as displays, backplanes, radio frequency identification (RFID), photovoltaics, lighting, disposable electronics, and memory sensors, as well as traditional thick film applications in which screen printing is used in the creation of PCBs, automobile heaters, EMI shielding, and membrane switches. There is tremendous interest in the field of RFID and printed electronics. This is because major retailers and institutions need to be able to more accurately and efficiently track inventory, and RFID and printed electronics are considered the ideal solution.
Among various electrically conductive nano particles, silver is commonly considered the material of choice for RFID antennas; but, nano silver particles are very expensive. A carbon-based ink typically contains particles of graphite, amorphous carbon, or carbon black (CB) that are suspended in a binder/resin and a solvent. These inks are applied on a substrate surface via a number of deposition techniques, including brush painting, syringe application, inkjet printing, screen printing, and gas assisted spraying. The ink is allowed to dry and the resulting carbon-coated surface, if containing a binder or matrix resin, is subjected to a curing treatment. Further, printing RFID tags is seen as the most likely way to reduce their costs to a point where such tags can be widely used on cost sensitive items, such as food packages. Compared to micron-scaled particles, nano-scaled particles are more amenable to inkjet printing.
For printed electronics, all conventional carbon-based conductive particles have one or more shortcomings. For instance, graphite particles are too large in size to be inkjet printable; they easily clog up the dispensing nozzles. Carbon black is not sufficiently conducting and, hence, cannot be used alone as a conductive additive in an ink. Another class of carbon materials that can be inkjet printed is the carbon nano-tube (CNT) [Refs. 1-4]. CNTs, although relatively conducting, are prohibitively expensive. The production of CNTs necessarily involves the use of heavy metal elements as catalysts that are undesirable in many applications and must be removed. The CNTs that contain catalysts tend to undergo sedimentation in a dispersing liquid, which is a highly undesirable feature in a conductive ink. Further, CNTs tend to aggregate together and get entangled with one another due to their high length-to-diameter aspect ratio, making it difficult to disperse CNTs in water, organic solvents, and polymer matrices (for forming nanocomposites). The aggregation and entanglement of CNTs also dramatically increase the viscosity of the dispersing liquid [e.g., Refs. 5 and 6], to the extent that inkjet printing of CNT inks is possible only when an exceedingly low CNT concentration is involved. Similarly, processing of CNT-resin nanocomposite is not possible with melt mixing/molding (e.g., via extrusion or injection molding) when CNT loading exceeds 5% by weight [Refs. 7 and 8].
Therefore, there is a need for nano particle-containing conductive inks that exhibit the following features: (1) the conductive additives are much less expensive than CNTs; (2) the inks are printable, preferably inkjet printable using a conventional, low-cost printhead; (3) the additives and the resulting printed elements are highly conductive, electrically and/or thermally; (4) the additives can be readily dispersed in a wide range of liquid mediums and do not form a sediment; and (5) the inks can contain a high conductive additive content so that a desired amount or thickness of conductive elements can be dispensed and deposited onto a substrate in one pass or few passes (to avoid or reduce the need for repeated printing passes or overwrites). It is of interest to note that high thermal conductivity is a desirable feature of an additive for microelectronic packaging applications since modern microelectronic devices, when in operation, are generating heat at an ever increasing rate. An additive with a high thermal conductivity provides a more efficient thermal management material.